Add extra_args test

Co-authored-by: Min RK <benjaminrk@gmail.com>

tests/unit/test_app.py

@@ -78,6 +78,20 @@ def test_local_dir_image_name(repo_with_content):
     )
 
 
+def test_extra_buildx_build_args(repo_with_content):
+    upstream, sha1 = repo_with_content
+    argv = ['--DockerEngine.extra_buildx_build_args=--check', upstream]
+    app = make_r2d(argv)
+    with patch("repo2docker.docker.execute_cmd") as execute_cmd:
+        app.build()
+        
+    args, kwargs = execute_cmd.call_args
+    cmd = args[0]
+    assert cmd[:3] == ["docker", "buildx", "build"]
+    # make sure it's inserted before the end
+    assert "--check" in cmd[:-1]
+
+
 def test_run_kwargs(repo_with_content):
     upstream, sha1 = repo_with_content
     argv = [upstream]